Network Working Group                                        R. Aggarwal
Internet Draft                                          Juniper Networks
Expiration Date: July August 2010
                                                           J. L. Le Roux
                                                          France Telecom

                                                        January 29,

                                                       February 25, 2010

                 MPLS Upstream Label Assignment for LDP

                  draft-ietf-mpls-ldp-upstream-05.txt

                  draft-ietf-mpls-ldp-upstream-06.txt

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Abstract

   This document describes procedures for distributing upstream-assigned
   labels for Label Distribution Protocol (LDP). It also describes how
   these procedures can be used for avoiding branch LSR traffic
   replication on a LAN for LDP point-to-multipoint (P2MP)LSPs.

Table of Contents

 1          Specification of requirements  .........................   3
 2          Introduction  ..........................................   3
 3          LDP Upstream Label Assignment Capability  ..............   3
 4          Distributing Upstream-Assigned Labels in LDP  ..........   4
 4.1        Procedures  ............................................   5
 5          LDP Tunnel Identifier Exchange  ........................   6
 6          LDP Point-to-Multipoint LSPs on a LAN  .................   7
 7          IANA Considerations  ...................................   9
 8          Acknowledgements  ......................................          Security Considerations  ...............................   9
 9          Acknowledgements  ......................................  10
10          References  ............................................  10
 9.1
10.1        Normative References  ..................................  10
 9.2
10.2        Informative References  ................................  10
10
11          Author's Address  ......................................  11
1. Specification of requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2. Introduction

   This document describes procedures for distributing upstream-assigned
   labels [RFC5331] for Label Distribution Protocol (LDP). These
   procedures follow the architecture for MPLS Upstream Label Assignment
   described in [RFC5331].

   This document describes extensions to LDP that a LSR can use to
   advertise to its neighboring LSRs whether the LSR supports upstream
   label assignment.

   This document also describes extensions to LDP to distribute
   upstream-assigned labels.

   The usage of MPLS upstream label assignment using LDP for avoiding
   branch LSR traffic replication on a LAN for LDP P2MP LSPs [MLDP] is
   also described.

3. LDP Upstream Label Assignment Capability

   According to [RFC5331], upstream-assigned label bindings MUST NOT be
   used unless it is known that a downstream LSR supports them. This
   implies that there MUST be a mechanism to enable a LSR to advertise
   to its LDP neighbor LSR(s) its support of upstream-assigned labels.

   A new Capability Parameter, the LDP Upstream Label Assignment
   Capability, is introduced to allow an LDP peer to exchange with its
   peers, its support of upstream label assignment. This parameter
   follows the format and procedures for exchanging Capability
   Parameters defined in [LDP-CAP]. [RFC5561].

   Following is the format of the LDP Upstream Label Assignment
   Capability Parameter:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|0| Upstream Lbl Ass Cap(IANA)|      Length (= 1)             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1| Reserved    |
      +-+-+-+-+-+-+-+-+

   If a LSR includes the Upstream Label Assignment Capability in LDP
   Initialization Messages it implies that the LSR is capable of both
   distributing upstream-assigned label bindings and receiving upstream-
   assigned label bindings. The reserved bits MUST be set to zero on
   transmission and ignored on receipt. The Upstream Label Assignment
   Capability Parameter can be exchanged only in LDP initialization
   messages.

4. Distributing Upstream-Assigned Labels in LDP

   An optional LDP TLV, Upstream-Assigned Label Request TLV, is
   introduced.  This TLV MUST be carried in a Label Request message if
   an upstream-assigned label is being requested.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Upstream Ass Lbl Req (TBD)|      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Reserved                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An optional LDP TLV, Upstream-Assigned Label TLV is introduced to
   signal an upstream-assigned label. Upstream-Assigned Label TLVs are
   carried by the messages used to advertise, release and withdraw
   upstream assigned label mappings.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|0| Upstream Ass Label (TBD)  |      Length                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Reserved                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Label                                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Label

   This is a 20-bit label value as specified in [RFC3032] represented as
   a 20-bit number in a 4 octet field.

4.1. Procedures

   Procedures for Label Mapping, Label Request, Label Abort, Label
   Withdraw and Label Release follow [RFC3036] [RFC5036] other than the
   modifications pointed out in this section.

   A LDP LSR MUST NOT distribute the Upstream Assigned Label TLV to a
   neighboring LSR if the neighboring LSR had not previously advertised
   the Upstream Label Assignment Capability in its LDP Initialization
   messages.  A LDP LSR MUST NOT send the Upstream Assigned Label
   Request TLV to a neighboring LSR if the neighboring LSR had not
   previously advertised the Upstream Label Assignment Capability in its
   LDP Initialization messages.

   As described in [RFC5331] the distribution of upstream-assigned
   labels is similar to either ordered LSP control or independent LSP
   control of the downstream assigned labels.

   When the label distributed in a Label Mapping message is an upstream-
   assigned label, the Upstream Assigned Label TLV MUST be included in
   the Label Mapping message. When a LSR receives a Label Mapping
   message with an Upstream Assigned Label TLV and it does not recognize
   the TLV, it MUST generate a Notification message with a status code
   of "Unknown TLV" [RFC3036]. [RFC5036]. If it does recognize the TLV but is
   unable to process the upstream label, it MUST generate a Notification
   message with a status code of "No Label Resources". If the Label
   Mapping message was generated in response to a Label Request message,
   the Label Request message MUST contain an Upstream Assigned Label
   Request TLV. A LSR that generates an upstream assigned label request
   to a neighbor LSR, for a given FEC, MUST NOT send a downstream label
   mapping to the neighbor LSR for that FEC unless it withdraws the
   upstream-assigned label binding. Similarly if a LSR generates a
   downstream assigned label request to a neighbor LSR, for a given FEC,
   it MUST NOT send an upstream label mapping to that LSR for that FEC,
   unless it aborts the downstream assigned label request.

   The Upstream Assigned Label TLV may be optionally included in Label
   Withdraw and Label Release messages that withdraw/release a
   particular upstream assigned label binding.

5. LDP Tunnel Identifier Exchange

   As described in [RFC5331] an upstream LSR Ru MAY transmit a MPLS
   packet, the top label of which (L) is upstream-assigned, to a
   downstream LSR Rd, by encapsulating it in an IP or MPLS tunnel. In
   this case the fact that L is upstream-assigned is determined by Rd by
   the tunnel on which the packet is received. There must be a mechanism
   for Ru to inform Rd that a particular tunnel from Ru to Rd will be
   used by Ru for transmitting MPLS packets with upstream-assigned MPLS
   labels.

   When LDP is used for upstream label assignment, the Interface ID TLV
   [RFC3472] is used for signaling the Tunnel Identifier.  If Ru uses an
   IP or MPLS tunnel to transmit MPLS packets with upstream assigned
   labels to Rd, Ru MUST include the Interface ID TLV in the Label
   Mapping messages along with the Upstream Assigned Label TLV.  The
   IPv4 Next/Previous Hop Address and the Logical Interface ID fields in
   the Interface ID TLV SHOULD be set to 0 by the sender and ignored by
   the receiver.

   Four new Interface ID TLVs are introduced to support RSVP-TE P2MP
   LSPs, LDP P2MP LSPs, IP Multicast Tunnels and context labels. The TLV
   value acts as the tunnel identifier.

   1. RSVP-TE P2MP LSP TLV. Type = TBD. Value of the TLV is
    <Extended Tunnel ID, Reserved, Tunnel ID, P2MP ID> as carried in the
   RSVP-TE P2MP LSP SESSION Object [RFC4875]. The TLV value identifies
   the RSVP-TE P2MP LSP. It allows Ru to tunnel an "inner" LDP P2MP LSP,
   the label for which is upstream assigned, over an "outer" RSVP-TE
   P2MP LSP that has leaves <Rd1...Rdn>. The P2MP LSP IF_ID TLV allows
   Ru to signal to <Rd1...Rdn>  the binding of the inner LDP P2MP LSP to
   the outer RSVP-TE P2MP LSP. The control plane signaling between Ru
   and <Rd1...Rdn> for the inner P2MP LSP uses targeted LDP signaling
   messages

   2. LDP P2MP LSP TLV. Type = TBD. Value of the TLV is the LDP P2MP FEC
   as defined in [MLDP]. The TLV value identifies the LDP P2MP LSP. It
   allows Ru to tunnel an "inner" LDP P2MP LSP, the label for which is
   upstream assigned, over an "outer" LDP P2MP LSP that has leaves
   <Rd1...Rdn>. The P2MP LSP IF_ID TLV allows Ru to signal to
   <Rd1...Rdn>  the binding of the inner LDP P2MP LSP to the outer LDP-
   P2MP LSP. The control plane signaling between Ru and <Rd1...Rdn> for
   the inner P2MP LSP uses targeted LDP signaling messages

   3. IP Multicast Tunnel TLV. Type = TBD. In this case the TLV value is
   a <Source Address, Multicast Group Address> tuple. Source Address is
   the IP address of the root of the tunnel i.e. Ru, and Multicast Group
   Address is the Multicast Group Address used by the tunnel.

   4. MPLS Context Label TLV. Type = TBD. In this case the TLV value is
   a <Source Address, MPLS Context Label> tuple. The Source Address
   belongs to Ru and the MPLS Context Label is an upstream assigned
   label, assigned by Ru. This allows Ru to tunnel an "inner" LDP P2MP
   LSP, the label of which is upstream assigned, over an "outer" one-hop
   MPLS LSP, where the outer one-hop LSP has the following property:

     + The label pushed by Ru for the outer MPLS LSP is an upstream
       assigned context label, assigned by Ru. When <Rd1...Rdn> perform
       a MPLS label lookup on this label a combination of this label and
       the incoming interface MUST be sufficient for <Rd1...Rdn> to
       uniquely determine Ru's context specific label space to lookup
       the next label on the stack in. <Rd1...Rdn> MUST receive the data
       sent by Ru with the context specific label assigned by Ru being
       the top label on the label stack.

   Currently the usage of the context label TLV is limited only to LDP
   P2MP LSPs on a LAN as specified in the next section. The context
   label TLV MUST NOT be used for any other purposes.

   Note that when the outer P2MP LSP is signaled with RSVP-TE or MLDP
   the above procedures assume that Ru has a priori knowledge of all the
   <Rd1, ... Rdn>. In the scenario where the outer P2MP LSP is signaled
   using RSVP-TE, Ru can obtain this information from RSVP-TE. However,
   in the scenario where the outer P2MP LSP is signaled using MLDP, MLDP
   does not provide this information to Ru. In this scenario the
   procedures by which Ru could acquire this information are outside the
   scope of this document.

6. LDP Point-to-Multipoint LSPs on a LAN

   This section describes one application of upstream label assignment
   using LDP. Further applications are to be described in separate
   documents.

   [MLDP] describe how to setup P2MP LSPs using LDP. On a LAN the
   solution relies on "ingress replication". A LSR on a LAN, that is a
   branch LSR for a P2MP LSP, (say Ru) sends a separate copy of a packet
   that it receives on the P2MP LSP to each of the downstream LSRs on
   the LAN (say <Rd1...Rdn> that are adjacent to it in the P2MP LSP.

   It is desirable for Ru to send a single copy of the packet for the
   LDP P2MP LSP on the LAN, when there are multiple downstream routers
   on the LAN that are adjacent to Ru in that LDP P2MP LSP. This
   requires that each of <Rd1...Rdn> must be able to associate the label
   L, used by Ru to transmit packets for the P2MP LSP on the LAN, with
   that P2MP LSP. It is possible to achieve this using LDP upstream-
   assigned labels with the following procedures.

   Consider a LSR Rd that receives the LDP P2MP FEC [MLDP] from its
   downstream LDP peer. Further the upstream interface to reach LSR Ru
   which is the next-hop to the P2MP LSP root address, Pr, in the LDP
   P2MP FEC, is a LAN interface, Li. Further Rd and Ru support upstream-
   assigned labels.  In this case Rd instead of sending a Label Mapping
   message as described in [MLDP] sends a Label Request message to Ru.
   This Label Request message MUST contain an Upstream Assigned Label
   Request TLV.

   Ru on receiving this message sends back a Label Mapping message to Rd
   with an upstream-assigned label. This message also contains a MPLS
   Context Label TLV, as described in the previous section, with the
   value of the MPLS label set to a value assigned by Ru on inteface Li
   as specified in [RFC5331].  Processing of the Label Request and Label
   Mapping messages for LDP upstream-assigned labels is as described in
   section 4.2. If Ru receives a Label Request for an upstream assigned
   label for the same P2MP FEC from multiple downstream LSRs on the LAN,
   <Rd1...Rdn>, it MUST send the same upstream-assigned label to each of
   <Rd1...Rdn>.

   Ru transmits the MPLS packet using the procedures defined in
   [RFC5331] and [RFC5332]. The MPLS packet transmitted by Ru contains
   as the top label the context label assigned by Ru on the LAN
   interface, Li. The bottom label is the upstream label assigned by Ru
   to the LDP P2MP LSP. The top label is looked up in the context of the
   LAN interface, Li, [RFC5331] by a downstream LSR on the LAN. This
   lookup enables the downstream LSR to determine the context specific
   label space to lookup the inner label in.

   Note that <Rd1...Rdn> may have more than one equal cost next-hop on
   the LAN to reach Pr. It MAY be desirable for all of them to send the
   label request to the same upstream LSR and they MAY select one
   upstream LSR using the following procedure:

   1. The candidate upstream LSRs are numbered from lower to higher IP
   address

   2. The following hash is performed: H = (Sum Opaque value) modulo N,
   where N is the number of candidate upstream LSRs. Opaque value is
   defined in [MLDP] and comprises the P2MP LSP identifier.

   3. The selected upstream LSR U is the LSR that has the number H.

   This allows for load balancing of a set of LSPs among a set of
   candidate upstream LSRs, while ensuring that on a LAN interface a
   single upstream LSR is selected. It is also to be noted that the
   procedures in this section can still be used by Rd and Ru if other
   LSRs on the LAN do not support upstream label assignment. Ingress
   replication and downstream label assignment will continue to be used
   for LSRs that do not support upstream label assignment.

7. IANA Considerations

   This document defines a new LDP Upstream Label Assignment Capability
   Parameter. IANA is requested to assign the value 0x0507 to this
   Parameter.

   This document defines a new LDP Upstream-Assigned Label Request TLV, IANA is
   requested to assign the type value of 0x204 to this TLV.

   This document defines a new LDP Upstream-Assigned Label Request TLV,
   IANA is requested to assign the type value of 0x205 to this TLV.

   This document defines four new Interface ID TLVs:

     - RSVP-TE P2MP LSP TLV

     - LDP P2MP LSP TLV

     - IP Multicast Tunnel TLV

     -  MPLS Context Label TLV

   These values are assigned from the Interface_ID Type space defined in
   [RFC3471]. IANA is requested to assign the type values of these TLVs. value 6 to RSVP-TE
   P2MP LSP TLV, type value 7 to LDP P2MP LSP TLV, type value 8 to IP
   Multicast Tunnel TLV and type value 9 to MPLS Context Label TLV.

8. Security Considerations

   The security considerations discussed in RFC 5331 and RFC 5332 apply
   to this document.

   More detailed discussion of security issues that are relevant in the
   context of MPLS and GMPLS, including security threats, related
   defensive techniques, and the mechanisms for detection and reporting,
   are discussed in "Security Framework for MPLS and GMPLS Networks
   [MPLS-SEC].

9. Acknowledgements

   Thanks to Yakov Rekhter for his contribution. Thanks to Ina Minei and
   Thomas Morin for their comments. The hashing algorithm used on LAN
   interfaces is taken from [MLDP].

9.

10. References

9.1.

10.1. Normative References

   [RFC3031] "MPLS Architecture", E. Rosen, A. Viswanathan, R. Callon,
   RFC 3031.

   [RFC5331] R. Aggarwal, Y. Rekhter, E. Rosen, "MPLS Upstream Label
   Assignment and Context Specific Label Space", RFC5331

   [RFC5332] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter, RFC5332

   [RFC2119] "Key words for use in RFCs to Indicate Requirement
   Levels.", Bradner, March 1997

   [RFC3472] Ashwood-Smith, P. and L. Berger, Editors, " Generalized
   Multi-Protocol Label Switching (GMPLS) Signaling - Constraint-based
   Routed Label Distribution Protocol (CR-LDP) Extensions", RFC 3472,
   January 2003.

   [RFC3471] Berger, L. Editor, "Generalized Multi-Protocol Label
   Switching (GMPLS) Signaling Functional Description", RFC 3471 January
   2003.

   [RFC3036]

   [RFC5036] L. Andersson, et. al., "LDP Specification", January 2001.

9.2. RFC5036.

10.2. Informative References

   [MVPN] E. Rosen, R. Aggarwal [Editors], "Multicast in BGP/MPLS VPNs",
   draft-ietf-l3vpn-2547bis-mcast-08.txt

   [RFC4875] R. Aggarwal, D. Papadimitriou, S. Yasukawa [Editors],
   "Extensions to RSVP-TE for Point to Multipoint TE LSPs", RFC 4875

   [MLDP] I. Minei et. al, "Label Distribution Protocol Extensions for
   Point-to-Multipoint and Multipoint-to-Multipoint Label Switched
   Paths", draft-etf-mpls-ldp-
   p2mp-07.txt

   [LDP-CAP] draft-ietf-mpls-ldp-p2mp-08.txt

   [RFC5561] B. Thomas, et. al., K. Raza, S. Aggarwal, R. Aggarwal, JL. Le Roux,
   "LDP Capabilities", draft-thomas-mpls-
   ldp-capabilities-04.txt

10. RFC5561

   [MPLS-SEC] L. fang, ed, "Security Framework for MPLS and GMPLS
   Networks", draft-ietf-mpls-mpls-and-gmpls-security-framework-07.txt

   [RFC3032] E. Rosen et. al, "MPLS Label Stack Encoding", RFC 3032

11. Author's Address

   Rahul Aggarwal
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA 94089
   Phone: +1-408-936-2720
   Email: rahul@juniper.net

   Jean-Louis Le Roux
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex
   France
   E-mail: jeanlouis.leroux@orange-ftgroup.com